Section 3.4. Backing Up with the dump Utility
3.4. Backing Up with the dump UtilityFor many environments, dump may be all you need to ensure good-quality backups. There's a lot of controversy surrounding dump, though, stemming from the fact that it doesn't access the data through the filesystem the way most other backup utilities do. dump accesses the filesystem device directly. This is why it can back up files without changing their access times. However, it's also why the manpages for dump have always said to unmount filesystems prior to backing them up. Of course, no one ever does that, hence the controversy.
To use dump and restore for regular system backups, you need to understand the following:
The first thing to understand is what your dump command is and what its options are. See Table 3-1 for a listing of dump commands on various Unix versions. The following section is essentially a unified manpage for these dump-like commands on specific operating systems.
3.4.1. Syntax of the dump CommandLet's start with the basic dump command: # dump levelunbdsf blkg-factor density size device-name file_system The following are examples of running this command:
The preceding commands use three options (0, u, and n) that do not require arguments and four options (b, d, s, and f) that require a "companion" argument. The dump command accepts as its first argument a list of options, then each option's argument is placed on the command line in the same order in which the options are listed. Figure 3-2 illustrates how the dump command options relate to their companion arguments. Figure 3-2. Sample dump command 3.4.2. The Options to the dump CommandThe dump utility has seven main options that are available on most platforms:
If you are using dump for regular system backups, you should be using most of the preceding options. It is important to note that many of these options have default values, eliminating the need to specify that option and its argument in the dump command. For example, the default backup level is usually 9. The problem with the default values is that they vary between operating systems and may also vary even on the same operating system, depending on factors such as media type. It is better to specify each of these options the same way on all your dump backups to simplify making restores at a later date.
3.4.2.1. Specifying a complete or incremental backup (09)The first argument that you can specify is the dump level; you can use any number from 0 to 9. (See Chapter 2 for an explanation of backup levels.) Incremental dumps refer to the dumpdates file for the date of the last lower-level backup. (This file is discussed in the section "Updating the dumpdates file (u)" later in this chapter.) For example, if you are performing a level 5 backup, dump backs up all files that have changed since the last backup that was level 4 or lower. It gets the date of this backup from dumpdates (usually /etc/dumpdates). Since the dumpdates file is needed for incremental backups, you must use the u option to update it. 3.4.2.2. Specifying a blocking factor (b)The b option specifies the number of blocks to write in a single output operation. This refers to the number of physical blocks. The size of the entire block that dump writes depends on the size of the physical block multiplied by the blocking factor. For most versions of Unix, the physical block size for dump is 1024 bytes. So, if you specify a blocking factor of 10, the size of the actual block that dump writes is 10,240, or 10 K. This option is not available on SCO.
3.4.2.3. Updating the dumpdates file (u)The u option causes dump to update the dumpdates file for the filesystem that you backed up. (The dumpdates file is usually /etc/dumpdates, but is /var/adm/dumpdates on HP-UX 10.x.) This is a plain-text file that lists each filesystem's raw device and the date that the last backup of each level was taken on that device. Here is an example /etc/dumpdates file taken from a Solaris box: /dev/rdsk/c0t1d0s0 0 Sun Apr 30 23:07:22 2006 /dev/rdsk/c0t1d0s0 1 Wed May 3 02:49:51 2006 /dev/rdsk/c0t3d0s0 0 Sat May 20 00:31:49 2006 /dev/rdsk/c0t3d0s0 1 Mon May 29 01:33:33 2006 /dev/rdsk/c0t3d0s0 5 Wed May 31 00:28:14 2006 You can see that device c0t1d0s0 had a level 0 backup on April 30, and a level 1 backup on May 3, 2006. Device c0t3d0s0 had a level 0 backup on May 20, a level 1 on May 29, and a level 5 on May 31. There are a few important things to note about the dumpdates file. The first time you run dump on a system, you must first create an empty dumpdates file, and it must be owned by root. If it is not there or is not owned by root, dump does not create it. Your dump continues, but it will complain. Note that dumpdates is updated only if the entire dump completes successfully. If any errors cause dump to abort, dumpdates is not updated. This means that it is a good file to use for an automated script that checks to see if your dumps worked. The following list shows the various names and locations of the dumpdates file:
You might not want to use the u option when making a special "one-time" backup volume, because doing so changes the behavior of other backups. For example, if you are making a one-time level 0 backup for someone and use the u option, your automated level 1 backups will reference that level 0 backup that has been given to someone else and is not a part of your normal backup pool.
3.4.2.4. Notifying your backup operators (n)The n option causes dump to notify everyone in the operator group, as specified in the /etc/group file, if a dump backup requires attention. This notification looks similar to a wall message. (This option is not available on SCO.) A dump backup may require attention when any of the following occurs:
3.4.2.5. Specifying density and size (d and s)The density (d) and size (s) options do not affect how data is written to the backup media. The dump command uses them only to determine how much data can fit on a given volume and to determine when it has reached the logical-end-of-tape (LEOT, or the point at which dump thinks the volume is full) before it reaches the physical-end-of-tape (PEOT). dump then prompts the operator to switch volumes. The logic behind this is to keep the volume from hitting PEOT, because older versions of dump do not handle this well. Here is a quick explanation of these two flags:
In actual practice, these options are very difficult to use and yield very little value. Most people fake out dump using values that make dump think it will never run out of tape. This causes dump to use the entire volume and lets it discover the PEOT if or when it gets that far. There are many reasons for this:
Adding compression into the calculation really complicates the process, since compression is one area in which the phrase "your mileage may vary" really applies.
Figure 3-3. Example of a multiple-volume dump backup 3.4.2.6. Do I have to use the s and d options?A few newer versions of dump have done away with these options and provided a new size in kilobytes option you can use to specify the size of the volume in kilobytes. Even so, I personally use the s and d options with every dump command I run so that I don't have to remember how different versions work. You will find this is a common theme throughout this book: the more things you can do the same everywhere, the fewer things you have to worry about. The more per-host and per-OS customization you do, the more trouble you can get into. (For example, the size in kilobytes option uses a different letter on each version of Unix that supports it!) In this case, using the archaic size and density options actually makes writing shell scripts much easier, because you can use the same options on most versions of Unix. What happens, then, if you don't use the s, d, or size in kilobytes options? On some Unix flavors, dump uses the default values for size and density (except for AIX, which has apparently done away with these options altogether). Unfortunately, the default values are usually set to work with a nine-track tape. (Solaris has changed its default values to be slightly more sensible.) If this happens, dump will think it needs several volumes. The output of dump looks something like the following: DUMP: Estimated 5860 blocks (3006KB) on 39.00 tapes. Notice that it thinks it's going to need 39 tapes. This is what can happen if you do not use the size and density options to specify the capacity of the volume. As mentioned before, you can easily disable this feature by setting these values to some ridiculously high figure so that dump never thinks that it has run out of tape. (I personally use numbers like 1,000,000 for both.) 3.4.2.7. Specifying a backup device file (f)The f option specifies the name of the backup device to which you are sending the data. (This "device," of course, could be either an actual tape device or a file sitting on a disk, optical platter, or CD.) If you are expecting to use the hardware compression feature of your tape drive, make sure that you choose a device that supports compression. If you want to send the data to a drive on another system, use the format remote_system_name:device. Most versions of Unix support using remote devices in dump, as long as you're alright with using rsh as an authentication mechanism.
Remote devices require that the host with the remote device trust this host via the /.rhosts file. If you try to use a remote device from a nontrusted system, you might get the dreaded message: Permission Denied To test if you are a trusted host, try issuing the following command as root: # rsh remote_system uname -a If it does not work, you need to put a line with this system's name in the remote system's ~root/.rhosts file. Unfortunately, in today's mixed environments, you don't always know what other systems think a particular system's name is. The remote system might be using DNS, NIS, or a local hosts file. When you rsh to a system, it initially sees you as an IP address. It then does a gethostbyaddr( ) and tries to resolve that address into a name. Depending on how your particular system is set up, it may consult DNS, NIS, or the local /etc/hosts file; the order in which it consults these sources also varies with your setup. If it uses the local hosts file or NIS for address resolution, it may or may not appear with a fully qualified domain name such as apollo.domain.com. If it uses DNS, it appears with the fully qualified domain name. It is important to know this because this is the name you must put into the /.rhosts file. Suppose your system is called apollo, and the remote system is elvis. If you want to rsh from apollo to elvis, you should try the easy step first. On elvis, enter this command: $ echo apollo >>/.rhosts If that doesn't work, apollo appears as something else to elvis (e.g., apollo.domain.com). To find out for sure, you can telnet to elvis from apollo, then use commands such as last, who, tty, or netstat to look at the field that lists the system from which you came. If it turns out to be apollo.domain.com, put that into the /.rhosts file on elvis. (For example, at one client site, it appears as apollo.DOMAIN.COM.) Once you have put the correct name in /.rhosts, rsh should work. 3.4.2.8. Displaying which filesystems need to be backed up (W and w)The W and w dump options are available on most Unix systems and display information about which filesystems need to be backed up. Usually, the w option displays information on all filesystems, while the W option lists only those filesystems that need to be backed up, based on the backup level you have chosen. These options have slight variations between Unix flavors, so read the appropriate manpage. 3.4.2.9. Interesting options for Solaris's ufsdump utilitySolaris's ufsdump has a few options not found in other versions of Unix. It supports the l (autoloader), o (offline), a (archive file), and v (verify) options:
3.4.3. What a dump Backup Looks LikeThis section explains one primary difference between dump and its cousins, tar and cpio. dump writes a table of contents at the beginning of each volume while tar and cpio do not. 3.4.3.1. dump records an index on the volumeThe index is read during an interactive restore, allowing you to run commands such as cd and ls on this table of contents, viewing and selecting files that you want for the restore. (The restore utility is discussed later in this chapter.) This interactive restore feature is one of restore's biggest advantages over tar and cpio. Note one important thing about this index: it is made at the beginning of the backup, before it has tried to actually back up anything. The presence of the index makes the interactive restore efficient because you don't have to read the whole volume before you can see what's on it. However, the fact that it's created before the backup data is written, and possibly minutes or hours before the data is written to tape, means that files made during the backup are not included, and files deleted during the backup are listed on the index but are not actually on the volume. 3.4.3.2. Using the index to create a table of contentsYou can create a table of contents of a dump volume by physically reading the contents of the index that dump creates and seeing what dump intended to write to the volume. Also, it is important to mention that this reading of the volume in no way guarantees the integrity of the actual file on the volume any more than an ls -l on a file in a directory verifies its integrity. You may be wondering why this discussion is included here, in the section about dump; it is because making this table of contents should be a part of every dump backup that you take. Having said that, how do you create a table of contents of a dump file? First, what does "dump file" really mean? Perhaps an illustration would help; see Figure 3-4. Figure 3-4. The format of a dump tape A volume created by dump may have multiple dump files, sometimes called partitions, on it. Each file ends in an end-of-file (EOF) mark, symbolized in Figure 3-4 by shaded areas. You have two options if you want to obtain a table of contents for dump file 3 in Figure 3-4:
The first method is the easiest, because it involves only one step. The syntax of the command is as follows: $ restore tsbfy file blocking-factor device To read the third dump file on the tape with a blocking factor of 32, use the following command: $ restore tsbfy 3 32 /dev/rmt/0cbn Here's a list of the options used and what they do:
If you do choose to manually manipulate the tape, as in the second option, you need to be familiar with your Unix version's magnetic tape command. This is usually mt. It has five optionsstatus, rewind, offline, fsf, and fsrfour of which you might use when manipulating dump tapes. The format of the command is: $ mt -t device argument
Some versions of mt use a -f instead of a -t. The device argument is the no-rewind tape device that you are using, such as /dev/rmt/0n. Now specify one of the following for argument:
The following are examples of how to use the mt command. To rewind the tape /dev/rmt/0cbn, issue the command: # mt -t /dev/rmt/0cbn rewind To fast-forward the tape /dev/rmt/0cbn to the second file on the tape, issue the command: # mt -t /dev/rmt/0cbn fsf 1 To eject the tape /dev/rmt/0cbn, issue the command: # mt -t /dev/rmt/0cbn offline To get the status of the tape /dev/rmt/0cbn, issue the command: # mt -t /dev/rmt/0cbn status Once you have positioned the tape to the proper file, simply use the same restore command as before, leaving off the s option and its argument: $ restore tbfy 32 /dev/rmt/0cbn Whichever method you use, the table of contents is sent to standard output, which you should redirect into a file. One important thing to note about this output is that the name of the filesystem dumped to this volume is not in the output. This table of contents is relative to that filesystem, whatever its name was. For example, if you backed up /var, and you were looking for /var/adm/messages, the output would look something like this: 345353 ./adm/messages I recommend that you create a table of contents for each dump volume when you make it and store this output in a file that matches the name of the volume. Obviously, you should use a unique name, like: ./dump.system.filesystem.level0.May19.2006 Saving tables of contents in this way is very handy when you're searching for a file and you can't seem to find it on any volume. A quick grep of all the dump files shows you which volume you need.
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